Acoustic Wave Device (AWD) based devices, and more commonly Surface Acoustic Wave (SAW) devices, provide high frequency devices with well controlled delay times or resonant frequencies in a compact structure. The manufacture of SAW devices is consistent with wafer processes developed for the semiconductor industry. SAW devices use the piezoelectric property of certain crystals to couple an electromagnetic signal to an acoustic wave. In the case of the SAW, the acoustic wave is trapped to the surface of the device, where it readily interacts with thin film structures such as reflectors and interdigital transducers. SAW devices have been prized for the past 40 years for their unparalleled ability to provide long time delay (microseconds) in a small size (millimeters) with low loss (single dB's) and to provide resonators with Q in excess of 10,000 at UHF radio frequencies.
There is widespread and growing interest in the use of SAW resonators and delay lines as sensors. These devices utilize at least one device which is made to be sensitive to a physical or chemical property of the environment.
Many quartz resonators change their operation parameters such as insertion loss, phase relationships, frequency, and similar characteristics, in response to changes in ambient temperature. Generally this phenomenon has been considered a nuisance for the design of stable SAW based filters and resonators, and significant research was performed to minimize the effects of temperature on SAW's.
For brevity, these specifications would provide example embodiments and discussion relating to a temperature sensor, utilizing SAW resonators as sensor elements, and frequency change ΔF as the sensed parameter. The skilled in the art would readily recognize however that the specifications and the claims extend to equivalent modifications of the teachings provided herein, utilizing delay lines or other SAW structures as sensor elements, and using propagation delay, phase differences, and the like as the sensed parameters.
Examples of research into extremely temperature stable frequency control and timing devices include, by way of example, U.S. Pat. No. 7,042,133 to Kanna, and U.S. Pat. No. 4,400,640 to Williams and Cho. However, orientations may also be chosen that are not temperature compensated and may be utilized for temperature measurements by sensing one of the temperature-dependent SAW operating parameters. Commonly, the sensed operating parameter is frequency.
Wireless sensors utilizing SAW's are well known in the art. When exposed to a pre-set signal from a transmitter, such sensors re-radiate Radio Frequency (RF) energy, or otherwise disrupt the RF energy field, in a manner that conveys information about the parameter they are designed to measure. The re-radiated energy or field disruption is received by a receiver which allows the information to be used. The excellent delay time or resonant Q of SAW devices allows the return signal to be examined microseconds after the transmitter has been silenced, affording exquisite signal to noise performance. This technology is colloquially known as a ‘passive device’ or ‘passive sensor’.
In “Passive Remote Sensing for Temperature and Pressure Using SAW Resonator Devices”, Buff et al. IEEE transactions on ultrasonics, ferroelectrics, and frequency control, vol. 45, no. 5, pp. 1388-1392, September 1998 Buff showed a SAW based temperature and pressure sensors that utilized two resonators as sensor elements. The resonators were deposited on a so-called 35° Rotated Y Cut (RYC) quartz crystal, with 35° angle between the plate normal axis and the crystallographic axis. Such differential schemes employ two nearly identical devices which share sensitivity to other ambient conditions but have different sensitivity to the desired measurement condition.
The shared sensitivity of the two elements is known as ‘common mode’, and, when properly balanced, it provides compensation against the undesired effects of non-related environmental conditions on the measurement of interest. For example, as the disturbing influence of changes in the antenna impedance cause significant inaccuracies in SAW based passive sensors, the differential design offers significant advantages, as the frequency response of both sensor elements shift essentially identically in response to antenna loading. Aging, package stress induced shifts and many manufacturing variations are also seen to be compensated in at least some well designed differential sensors.
Certain embodiments of differential temperature sensors utilize two sensor elements which do not reside on the same die, such as dies which come from wafers of different orientations, and the like. The sensor elements are placed in the same ambient conditions. These embodiments offer the advantage of small size: the overall dimensions may be made as small as twice the size of a single device. However, the manufacturing variations of the two devices are uncorrelated, requiring large tolerances of individual frequencies and offering poor correlation of environmental sensitivity to undesired influences. Monolithic sensors, which are sensors where the two sensing elements are manufactured on a single substrate, are more common. However the monolithic differential sensors of prior designs suffer from larger size and other disadvantages related to the angle between the two monolithic sensor elements. The efficacy of such common mode compensation depends entirely on the similarity of the two elements of the differential sensor, but the differences between the elements that afford differential temperature sensitivity also introduce variations in the sensitivity to common mode effects, impairing compensation.
U.S. Pat. No. 6,774,747 to Yamazaki et al. discloses a non-negligible metal film employed for the reflectors and transducers of the SAW resonator. It further discloses the use of singly-rotated quartz orientations from 23° to 45° with propagation directions between 40 and 49° and a metallization ratio less than 50%, more preferably 32%, and the thickness of the metal film being 6% of the acoustic wavelength. This resulted in lowering the temperature sensitivity for a singly rotated cut, offering improved frequency/temperature stability. Yamazaki also discloses a propagation direction for each quartz orientation that is also a function of metal thickness for which the frequency of a single resonator is optimally stable over temperature.
U.S. Pat. No. 7,042,133 to Kanna presents a plurality of resonators based on the structure of U.S. Pat. No. 6,774,747, the resonators having overlapping resonant frequency passbands and connected so as to form a single composite resonator with a single resonant frequency. The aggregate resonator of such a structure was found to have even better frequency/temperature stability. It should be noted that the device of Kanna is clearly distinguished from sensors (discussed below) wherein multiple resonators with non-overlapping passbands are employed in differential sensing schemes, either connected in parallel or individually instrumented. The goal of frequency control devices is to minimize dependency on temperature fluctuations while the temperature sensors rely primarily on such fluctuation and seeks to maximize it as long as it is predictable and preferably linear over the measured range.
The Buff et al. design used an inter-element angle of 35°. As a result of the large angle between the resonators, the die is actually required to be wider than it is long, a decided cost disadvantage to the separate die implementation. The manufacturing variations are well correlated only to the degree to which the angle between the two resonators is sufficiently small that their sensitivities of nominal frequency to metal thickness, line-width variations, and misalignment are all correlated; however, the requisite small angle between sensor elements typically results in low differential sensitivity. Other angles have been practiced, ranging between 15° in a torque and temperature sensor, 18° in a tire pressure and temperature sensor, and 35° in Buff's temperature sensor. The poor correlation of resonant frequencies of the two sensor elements in mass production when using large angles leads to one element or the other being outside of specification. If either element is out of specification, the entire die is discarded, so the disadvantage of larger size is compounded by increased yield losses.
In addition to a large die size and manufacturing variations of the nominal frequencies, the temperature sensitivity of the individual elements are also well correlated only if the angle between the two resonators is sufficiently small that their sensitivities of temperature coefficient to metal thickness, line-width variations, and misalignment are all correlated. Correlation of the temperature sensitivity of the elements over manufacturing variations requires relatively small angles; however, utilizing a small angle under the prior art would typically result in low differential sensitivity
Therefore there is a clear but as of yet unresolved need for a small and inexpensive, yet accurate SAW based sensor. Ideally such sensor would utilize a suitable substrate orientation angle, suitable metal thickness, and two carefully selected propagation directions. The ideal substrate would have a differential resonant frequency change responsive to temperature (TCdF) which is monotonic, and preferably linear, over the desired measurement range, and preferably have a relatively large slope for better resolution. Having a sufficiently low slope to operate over the desired temperature range while staying within an unregulated radio frequency band is also a desired characteristic. The ideal substrate would also be very stable regarding all other parameters. Most preferably, the frequency response to temperature change of each of the two sensor elements would be in opposite directions.